The present invention relates to a method and an apparatus for correcting thermal displacement of a machine tool. Specifically, the present invention relates to a method and an apparatus for correcting thermal displacement of a machine tool caused by heat generated by rotation of a spindle.
In general, parts of a machine tool can be heat sources. Heat sources include rolling bearings of a spindle. Such rolling bearings produce friction heat. Heat generated by heat sources of machine tool parts causes thermal displacement at the machine tool having the spindle. Thermal displacement of a machine tool adversely affects machining accuracy of the machine tool. Accordingly, various types of methods and apparatus for correcting thermal displacement have been conventionally proposed.
Japanese Patent No. 3151655 discloses a method for correcting thermal displacement that is performed on the precondition that thermal displacement is proportionate to the amount of increase of the difference between the temperature of a spindle and the room temperature. The correction method includes a step for measuring temperatures of various parts of a machine tool, and a step for filtering temperature data obtained through measuring to estimate the amount of thermal displacement in each part of the machine tool. The time constant of the filter is changed in accordance with a spindle rotation speed and the rotation time of the spindle at the specific spindle rotation speed. When the spindle rotation speed decreases, an intermediate value of the temperature is obtained by means of an arithmetic expression to quickly estimate the thermal displacement amount that corresponds to the decrease of the spindle rotation speed. Further, the difference between the measured temperature data and the intermediate value for estimation is calculated, and the calculated difference is deleted from the temperature data. The deleted amount is reduced on the first-order lag. Based on the input temperature thus obtained, thermal displacement amount is computed.
The thermal displacement correction method according to claim 2 of Japanese Laid-Open Patent Publication No. 11-338527 focuses on heat generation and heat radiation about a spindle. The method obtains heat accumulation in a spindle based on the heat generation amount and the heat radiation amount about the spindle. Further, based on the heat accumulation, the thermal displacement amount of the spindle is calculated. The thermal displacement correction is executed based on the thermal displacement amount.
As described above, Japanese Patent 3151655 corrects thermal displacement on the precondition that the thermal displacement is proportionate to the temperature of the machine tool. As long as the machine tool is in a steady state, thermal displacement is accurately corrected by the method. However, immediately after the spindle rotation speed changes, the changed time constant of the thermal displacement does not match with the change time constant of the measured temperature. Thus, the thermal displacement cannot be easily corrected in an appropriate manner. In Japanese Patent No. 3151655, the thermal displacement amount is computed directly from the measured temperature data. Therefore, the amount of adjustment of the filter needs to be estimated every time the conditions change. Also, when the spindle rotation speed changes as described above, particularly complicated processes need to be executed.
Japanese Laid-Open Patent Publication No. 11-338527 will now be discussed. Heat radiation effect of components about a spindle depends on the relative velocity between a cooling medium and cooled objects (the components about the spindle). In actuality, cooling of the components about the spindle (for example, bearings) is closely related to the spindle rotation speed. When the spindle rotation speed increases and the heat generation amount increases, accordingly, the heat radiation amount also increases. That is, the time constant when thermal displacement increases due to an increase in the spindle rotation speed varies depending on the amount of increase in the spindle rotation speed. Likewise, the time constant varies when thermal displacement decreases due to a decrease in the spindle rotation speed. In this manner, the conditions of both of the heat generation and heat radiation change according to changes in the spindle rotation speed. Therefore, the estimation of thermal displacement by separately obtaining heat generation and heat radiation is difficult and not practical.